Motif-specific, context-independent antibodies are useful in characterizing various forms of cellular regulation as well as serving to profile genome wide changes in cellular protein levels and protein modification. Identifying the targets of intracellular signaling cascades is of major importance in understanding cell growth, differentiation, and cell death. Protein kinase cascades relay information from the cell surface to multiple cellular compartments including the nucleus and more distant cell processes such as synapses (Karin et al., Curr. Opin. Cell. Biol. 6:415-424 (1994)).
Although a few targets of protein phosphorylation have been identified, most remain unknown, particularly those that regulate cell growth and differentiation. For example, the MAP kinase cascade is known to play an important role in the regulation of cell growth (Lewis et al., Adv. Cancer Res. 74:49-139 (1998), Crowley et al., Cell 77:841-852 (1994)). However, beyond a handful of substrates, few protein targets responsible for the diverse actions of the MAP kinase cascade have been identified (Fukunaga and Hunter, EMBO 16(8):1921-1933 (1997), Stukenberg et al., Curr. Biol. 7:338-348 (1997)).
Another example of cell signaling proteins are the 14-3-3 proteins, which represent a phylogenetically conserved family of phosphoserine binding proteins whose precise role in cell signaling has yet to be determined (Burbelo and Hall, Curr. Biol. 5(2):95-96 (1995)). These proteins represent a large fraction of total brain protein and are known to bind a wide variety of signaling molecules including: ras, raf, bad, cdc25, and many others (Yaffe et al., Cell 91:961-971 (1997)). Recently, it has been shown that 14-3-3 proteins bind specifically to phosphorylated sites on proteins with the following motif: RXRSXS*XP (SEQ ID NO: 146) where S* is phosphoserine and X represents any amino acid (Muslin et al., Cell 84:889-897 (1996), Yaffe et al. supra (1997)).
Similarly, histones have long been known to be modified by acetylation at specific lysine residues. Acetylation of lysine in histones is thought to reduce protein-DNA interactions and serve to open chromatin in regions undergoing transcription (Struhl, Genes & Development, 12:599-606 (1998)). Recently, other proteins associated with transcription complexes have been shown to be acetylated on lysine although the functional significance is unclear (Imhof et al., Curr. Biol. 7:689-692 (1997), Struhl supra (1998)).
Antibodies against phosphotyrosine have proven to be of great value in identifying and characterizing intracellular signaling mechanisms (Ross et al., Nature 294:654 (1981), Kozma et al., Method. Enzymol. 201:28 (1991), White and Backer, Method. Enzymol. 201:65 (1991), Kamps, Method. Enzymol. 201:101 (1991)). Their value derives from two properties; 1) their ability to discriminate whether or not a protein is tyrosine phosphorylated, and 2) their ability to react with a large variety of different proteins. These properties have proven invaluable in tracing intracellular signaling pathways and identifying new targets of activated tyrosine kinases.
Ideally, the most useful phosphotyrosine antibodies should be as general as possible, that is they should recognize phosphotyrosine independently of the protein sequences in which it is embedded (context independent) so as to allow detection of all possible phosphotyrosine residues. The most successful approaches for producing phosphotyrosine antibodies have utilized phosphotyrosine or phosphotyramine coupled via their free amino groups to keyhole limpet hemocyanin using hetero- or bifunctional crosslinking agents (Frackelton et al., Method. Enzymol. 201:79 (1991), White and Backer supra (1991), Wang, Method. Enzymol. 201:53 (1991), Kamps supra (1991)). Although currently produced polyclonal and monoclonal phosphotyrosine antibodies do recognize many different proteins, they often show crossreactivity with other phosphate containing compounds, for example, mononucleotides (Frackelton et al. supra (1991), Kamps supra (1991)). More importantly, most phosphotyrosine antibodies raised in this fashion display variable sequence reactivity, depending not only on the phosphorylated amino acid, but also upon the amino acid sequences surrounding phosphotyrosine. For example, the present inventors have observed that most phosphotyrosine antibodies do not recognize phosphotyrosine preceded by proline as found in the activation loop of JNK and hence do not react significantly with activated (tyrosine phosphorylated) JNK [(Tan et al. unpublished observations)]. The reason for the variable reactivity is likely due to the fact that the phosphotyrosine antigen is not presented directly to the immune system in the context of variable surrounding amino acids, but is instead presented as a hapten, inappropriately coupled to the KLH carrier via artificial linkages. This approach tends to produce antibodies that react well with phosphotyrosine but are sometimes blocked by surrounding amino acids as they are not present in the antigen.
Other approaches have utilized total cellular phosphotyrosine containing proteins as immungens (Glenney, Method. Enzymol. 201:92 (1991), Wang supra (1991)) with considerable success but the context-dependence of the resulting antibody specificities was not carefully determined, although antibodies raised in this fashion did react with a majority of tyrosine phosphorylated proteins. Estimates as to the fraction of tyrosine phosphorylated proteins detected range from 50% to 94% (Kamps supra (1991)).
Attempts to use the above mentioned techniques to produce similar antibodies for phosphoserine and phosphothreonine have met with limited success. Antibodies produced to date have limited crossreactivity and lower affinity for phosphoserine or phosphothreonine probably due to the poor immunogenicity of these phospho-amino acids compared to phosphotyrosine (Heffetz et al., Method. Enzymol. 201:44 (1991)). Context-dependence and low affinity have limited the utility of currently available phosphoserine and phosphothreonine antibodies, especially when compared to phosphotyrosine antibodies.
Site-specific phosphoserine and phosphothreonine antibodies were first described by Nairn et al. in 1982 and have proven to be highly useful tools to study protein phosphorylation (Czernik et al., Method. Enzymol. 201:264 (1991), Czernik et al., Neuroprot. 6:56-61 (1995)). One drawback of this type of antibody is that a different antibody needs to be produced for each site of interest. Clearly, development of antibodies that detect phosphoserine or phosphothreonine in a substantially context-independent fashion would be desirable for use in tracing serine/threonine kinase cascades and in defining their biological responses. Likewise, development of context-independent phosphotyrosine antibodies would overcome the limitations of currently available antibodies.
Motif-specific, context-independent antibodies would also be useful in identifying new targets of 14-3-3 action (i.e., other proteins phosphorylated at this motif) and in characterizing the protein kinases that phosphorylate these sites. Likewise antibodies reactive against acetylated lysine would serve as useful tools to study the functional significance of acetylation of histones.
Such antibodies can further be used as general reagents for detecting phosphorylation or other enzymatic modification in vitro, such as in high throughput kinase assays for drug screens, as a single antibody can be used to recognize many different phosphorylated substrates. Phosphotyrosine antibodies are currently employed in high throughput kinase assays to screen for selective, high affinity tyrosine kinase inhibitors. Compounds or drugs that block enzyme activity are detected by their ability to inhibit kinase activity as determined by a reduction of phosphotyrosine antibody binding to phosphorylated substrate. Similar assays can be set up to screen for pharmaceutically useful compounds using antibodies produced as described above for phosphoserine, phosphothreonine, or antibodies detecting other protein modifications.
Antibodies that detect short recurring motifs in a context-independent fashion will also be particularly useful in profiling genome wide changes in protein levels and protein modification. For example, the use of context-independent phosphothreonine antibodies and 2D gel electrophoresis to profile genome wide changes in protein phosphorylation (Patterson and Garrels, Cell Biology: A Laboratory Handbook 249-257 (1994), Academic Press) as the result of drug treatment or overexpression of a particular protein will undoubtedly prove useful in identifying potential drug-protein interactions and suggest new downstream targets for overexpressed proteins.